FIG. 1 shows part of a telecommunications network 10. The network 10 comprises a radio base station 12 (also known as a NodeB or an ENodeB) and a core network 14 which communicates with the radio base station 12. The radio base station 12 further communicates with a mobile terminal 16 (also known as a user equipment). In so-called uplink transmissions, data and control messages are transmitted from the mobile terminal 16 to the radio base station 12. In so-called downlink transmissions, data and control messages are transmitted from the radio base station 12 to the mobile terminal 16.
Various telecommunications standards exist to define communications within the network 10. An important part of such standards is the control of the transmission power for uplink transmissions.
In wideband code-division multiple access (WCDMA), fast uplink power control is based on measurements of the dedicated physical control channel (DPCCH) transmitted by the mobile terminal to the radio base station. Here, DPCCH pilot symbols are used by the radio base station, and transmission power control (TPC) commands are sent to the mobile terminal to adjust the transmission power of the mobile terminal based on the measured quality of those pilot symbols, e.g. the signal to interference and noise ratio (SINR). The power levels of other channels transmitted from the same mobile terminal are controlled using beta-factors defining fixed power offsets relative to the DPCCH. The DPCCH pilot symbols are sent continuously—or in the case of continuous packet connectivity (CPC), semi-continuously and rather frequently—by the mobile terminal, enabling good measurement quality. Further, the other channels (such as the dedicated physical data channel, or DPDCH) are transmitted on the same frequency of 5 MHz, such that a fixed offset is feasible. The DPCCH and DPCCH are mapped to orthogonal channelization codes (such as I and Q branches).
As shown in FIG. 1, in the long term evolution (LTE) standard (i.e. E-UTRAN as defined in Release 8 of the 3GPP specifications), two uplink channels are the physical uplink control channel (PUCCH), i.e. a control channel, and the physical uplink shared channel (PUSCH), i.e. a data channel. The transmit powers of these channels are controlled individually for each mobile terminal. This is necessary as the channels are allocated to different parts of the frequency bands and therefore have different inter-cell interference situations. Furthermore, PUSCH is multi-user orthogonal with frequency division multiplexing while PUCCH is code-divided, resulting in a loss in performance due to intra-cell interference. This intra-cell interference occurs due to non-orthogonality between codes in multi-path radio channels. It is thus very important to have an accurate power control on PUCCH when several mobile terminals transmit over the PUCCH simultaneously.
Control information is sent on PUCCH, such as HARQ ACK/NACK messages, channel state information (CSI) messages and scheduling requests. When the mobile terminal is scheduled PUSCH resources, this control information is sent on PUSCH. In Release 8 of the standard, a mobile terminal does not transmit PUCCH and PUSCH simultaneously.
The transmit powers of the PUCCH and the PUSCH are controlled by sending transmission power control (TPC) commands along with downlink transmissions. The TPC commands may, for example, be sent on the physical downlink control channel (PDCCH), see 3GPP TS 36.213. The PUCCH is transmitted within one resource block in two slots of a sub-frame, with one slot in each end of the frequency band achieving frequency diversity. The same power is applied on both PUCCH slots, which means that power control cannot follow fast multipath fading.
Channel quality measurements are an important part of determining appropriate transmit powers for the PUCCH and PUSCH. Two types of uplink reference signals are known by the radio base station, and can be used for channel estimation etc.
The first, known as a demodulation reference signal, is associated with transmission of PUSCH or PUCCH by individual mobile terminals. That is, the demodulation reference signal is individually scheduled for and transmitted by a mobile terminal as part of the PUSCH or PUCCH, depending on which channel is being transmitted. The second reference signal, known as a sounding reference signal, is not associated explicitly with PUCCH or PUSCH, but covers a wider range of frequencies, including that of the PUSCH. Sounding reference signals may be transmitted from more than one mobile terminal simultaneously. The sounding reference signal is used to improve the PUSCH channel measurement, for example to get a frequency selective measure over the PUSCH part of the frequency band.
A value of the TPC adjustment to the PUCCH can be achieved by estimating the path loss of the reference signals (also known as pilots) transmitted simultaneously on the PUCCH. When the path loss is large for a mobile terminal, that mobile terminal should be allowed to increase its transmitted power such that all users have approximately the same signal strength when received at the radio base station. Also the orthogonality and interference from other cells must be compensated for to achieve high performance and allow several users to transmit at the same time on PUCCH.
The straightforward solution is to directly measure the quality (for example, the signal-to-interference-and-noise ratio, SINR) of the received transmissions on PUCCH. The power adjustment is signalled to the mobile terminal by the use of TPC. However, when the interference from other mobile terminals is large, the path loss estimates become uncertain, and consequently the TPC commands can also become uncertain. When operating with a low information bitrate, the PUCCH has a low SINR, where it is more difficult to estimate the quality. The PUSCH, which is orthogonal and operates with a high information bitrate, preferably operates with a higher SINR.